Large generators are driven by a prime mover to produce a supply of electric energy. A synchronous generator rotor is energized by an exciter providing to the generator's field winding a supply of DC power effective to produce a magnetic field. An annular stator surrounding the rotor contains a plurality of windings in which electricity is induced by the rotating magnetic field.
Providing the supply of DC power to the rotor involves transferring the DC power from a stationary element to the rotating element. One method for transferring the DC power includes the use of slip-rings rotating with the rotor in combination with stationary brushes that contact the slip-rings. The use of slip-rings in this manner are subject to reliability and arcing problems. The arcing problems can present a hazard when the generator has to operate in volatile gas environments such as near oil and gas plants or in military applications
An improved technique for transferring power from the stationary element to the rotating element uses a brushless exciter in which a DC field is applied to a stationary exciter winding. One or more windings rotating with the rotor pass through the magnetic field produced by the stationary exciter winding thereby producing AC power. The exciter AC power is rectified in a rectifier located on the rotor to produce the required DC excitation. Also, wound rotor induction machines can be used with a rotating rectifier connected to the rotor windings.
When these synchronous generators make use of a superconductor in the rotor field, the electrical time constant of super cooled or high temperature superconducting (HTS) field windings in can be greater than one hour due to the fact that HTS field winding internal resistance approaches zero at HTS temperatures. In the generator output voltage regulation, a fast response of the current in the field windings is required to compensate any changes in load at the generator terminals. This requires that the exciter have a negative forcing function capability to de-excite the field windings when required by an output voltage regulator. For the case of static exciters, this is accomplished by using a stator mounted thyristor or silicon controlled rectifier (SCR) bridge connected to the rotor-mounted field winding via slip rings since a thyristor bridge can produce a negative DC voltage.
Unfortunately, in the case of brushless exciters, the typical rotating diode bridge used does not allow for the application of negative forcing voltage to the field coil. All proposed topologies require the use of force-commutated devices like SCR's, FET's, etc. mounted in the rotor to produce negative DC voltage across the field windings. Mounting and controlling the devices is challenging. Redundancy requirements add to the system complexity. Moreover, the number of control signals to be transmitted to the rotor increases with the addition of each semiconductor.
Accordingly, there is a need for an improved brushless exciter with less redundancy so as to reduce the complexity as well as the number of control signals transmitted to the rotor.